Although austenitic stainless steels have hitherto been used in the components of nuclear reactors, especially fast reactors which are required to have excellent high-temperature strength and resistance to neutron irradiation, they have limitations on irradiation resistance such as swelling resistance. On the other hand, ferritic stainless steels have the disadvantage of low high-temperature strength although they are excellent in irradiation resistance.
Therefore, oxide dispersion strengthened ferritic steels in which fine oxide particles are dispersed have been proposed as materials excellent in irradiation resistance and high-temperature strength. It is also known that in order to improve the strength of the oxide dispersion strengthened ferritic steels, it is effective to further finely disperse the oxide particles by adding Ti to the steels.
In particular, for improving the high-temperature creep strength of oxide dispersion strengthened ferritic steels, it is effective to make grain coarse and equiaxed in order to suppress grain-boundary slidings. As a method of obtaining such a coarse grain structure, there has been proposed, for example, a method wherein a sufficient amount of α to γ transformation is ensured by performing austenitization heat treatment which involves heating to a temperature of not less than the Ac3 transformation point and holding at this temperature, thereby causing austenitization to occur by phase transformation from α-phase to γ-phase, and after that, slow cooling is performed at a sufficiently low rate, i.e., at a rate of not more than the ferrite-forming critical rate so that a ferrite structure can be obtained by phase transformation from γ-phase to α-phase (refer to, for example, the Japanese Patent Laid-Open No. 11-343526/1999).
However, in the case where Ti is added to an oxide dispersion strengthened ferritic steel, there occurs a problem that Ti combines with C in the matrix to form a carbide, with the result that the C concentration in the matrix decreases and hence it is impossible to ensure a sufficient amount of α to γ transformation during austenitization heat treatment.
Namely, as described above, the heat treatment of an oxide dispersion strengthened ferritic steel to obtain a coarse grain structure involves slow cooling at a rate of not more than the ferrite-forming critical rate after obtaining γ-phase by performing austenitization heat treatment which involves heating to a temperature of not less than the Ac3 transformation point and holding at this temperature. However, since Ti has a strong affinity for C which is a γ-phase-forming element in the matrix, Ti and C combine to form a carbide. As a result, the C concentration in the matrix decreases, and a single phase of γ-phase is not formed even by the heat treatment at a temperature of not less than the Ac3 transformation point and untransformed α-phase is retained. For this reason, even when slow cooling is performed from γ-phase at a rate of not more than the ferrite-forming critical rate, for example, at a rate of not more than 100° C./hour, it follows that, due to the presence of retained a-phase, the a-phase which has transformed from γ-phase becomes a fine grain structure. Such a fine grain structure does not contribute to an improvement in high-temperature strength.